Effects of Temperature, Salinity, and Diet on the Growth and Survival of the Freshwater Rotifer Brachionus angularis

Abstract

We explored the possibilities of using the freshwater rotifer Brachionus angularis as a live food for small fishes cultured in fresh- or brackish waters. Brachionus angularis were collected from a reservoir for isolation and laboratory culture. Length and width of the lorica were <TEX>$102.3{\mu}m$</TEX> and <TEX>$76.6{\mu}m$</TEX>, respectively, and those of amictic eggs were <TEX>$64.4{\mu}m\;and\;47.9{\mu}m$</TEX>, respectively. When their growth rates were examined at six different temperatures, i.e., 15, 20, 25, 30, 35, and <TEX>$40^{\circ}C$</TEX>, the highest daily growth rate of 0.801 was observed at <TEX>$35^{\circ}C$</TEX>, and growth was lower with decreasing temperature. Adaptation to salinity change was evaluated with two different modes of salinity increase: step-wise elevation lasting for short durations of 5 to 30 min or a long duration of 24 h. With the short duration modes, no individuals survived salinity higher than 10 psu, and the number of live individuals did not increase throughout the experiment. However, in the 24-h elevation, the number of individuals increased when salinity was elevated by 1 to 2 psu per day for the first 2 or 3 days, while no increase in number occurred at salinity increments higher than 3 psu per day. In addition, to assess the effect of different diets, four single-component diets (Chlorella vulgaris, Nannochloris sp., baker's yeast, or dry yeast) and three combination diets (C. vulgaris + Nannochloris sp. + baker's yeast + dry yeast; C. vulgaris 70% + baker's yeast 30%; C. vulgaris 30% + baker's yeast 70%) were used. The specific growth rates of B. angularis fed combination diets were higher than those of rotifers fed any single-component diet, with the highest rate of 0.648 in B. angularis fed a mixture of C. vulgaris, Nannochloris sp., baker's yeast, and dry yeast, and the lowest rate of 0.200 in those fed dry yeast only. Our results indicate that the freshwater rotifer B. angularis can be used for seedling production of both freshwater and brackish-water fishes that require small (less than about <TEX>$120{\mu}m$</TEX>) live food during their early stages.